Transformers are used to change the voltage of an a.c. supply. At power stations, transformers are used to produce very high voltages before the electricity is transmitted to where it is needed through power lines (National Grid). Local transformers reduce the voltage to safer levels before the electricity is supplied to consumers.
A transformer consists of two separate coils wound around an iron core. When an alternating voltage is applied across one coil (the primary) an alternating voltage is produced across the other coil (secondary).
The a.c. current in the primary coil, is a constantly changing current which produces a changing magnetic field. This moving magnetic field is equilavent to the one around a moving magnet. The changing magnetic field will induce voltage in the secondary coil.
The higher the voltage, the smaller the current needed to transmit energy at the same rate. Less energy is wasted by heating up the power lines.
The voltages across the primary and secondary coils of a transformer are related as shown:
| voltage across primary (volt, V) | = | voltage across secondary (volt, V) |
| number of turns on primary | number of turns on secondary |
| Vp | = | Vs |
| Np | Ns |